Objectives: (1) Our most important aim is to complete the research dealing with separation and identification of the tryptophan photoproduct. Many of the methods have been worked out, and our assay system for testing fractions for toxicity are now reliable and routine. (2) We plan to screen a number of mutants to test which of these can be reverted to wildtype. This will allow us to characterize the nature of the mutation. Also, we plan to determine the frequency of mutation caused by the photoproduct in a turbidostatic culture. We will critically determine whether the photoproduct can produce deletions. (3) We plan to critically determine the photoproduct binds to DNA, to a nuclease, to a polymerase, or to another bio-molecule. (4) We plan to explore the significance of the photoproduct and of 300-400 nm light in eukaryotic systems, including mammalian cells in tissue culture.